High frequency attenuator



July 24, 1962 w. J. WILSON 3,046,505

HIGH FREQUENCY ATTENUATOR Filed Aug. 8, 1958 5 SheetsShe-et 1 William J.Wilson INVENTOR July 24, 1962 w. J. WILSON 3, 46,

HIGH FREQUENCY ATTENUATOR Filed Aug. 8, 1958 5 Sheets-Sheet 2 William J.Wilson INVENTOR July 24, 1962 w. J. wiLsN 3,046,505

HIGH FREQUENCY ATTENUATOR Filed Aug. 8, 1958 3 Sheets-Sheet 3 Fig. 7

I I4 Fig. 8 V

l4 Fig.9 .4 33

William J. Wilson IN VE NTOR quency of the line.

ware y 7 Filed Aug. 8, 1958, Ser. No. 753,983 4 Claims. (1.- 33381) The present invention relates to high frequency electric transmission line devices. More particularly, the present invention relates to transmission line attenuating devices and attenuating materials for use therein.

Many of the prior art devices' that provide a predetermined degree of attenuation for transmission lines, such as coaxial lines and waveguides, involve relatively complex, heavy, and expensive equipment. Another problem that is particularly perplexing in waveguide applications is the fact that such prior art devices have a band pass characteristic, i.e. ability to pass a range of frequencies, that is very limited.

A recent advance in the field of microwave attenuator technology isdescribed in the. copending application of Donald J. Somme'rs et al.,'Serial No. 484,406, filed January 27, 1955, which issuedflocto ber 20, 1959,. as, Patent No. 2,909,736. This device essentially comprises'a pair planar card of attenuating material between the inner'signal energy carrying conductors. lished thatthe principle of attenuation in a device of this It is now well estab character is based upon distortion and concentration of --the fringe fields produced by splitting the inner conductor. The spacing between the inner conductors is limited because of the requirement that the outer or ground plane conductors be spaced apart substantially less than onequarter of a wavelength at the highest operating fre- In the above-mentioned application of Sommers et al.,. an attenuating card comprising a granular resistive material provides a degree of attenuation which varies inversely as the resistance of the'card.

One means of decreasing the resistance of the card is by increasing its thickness; however, in view of the quarter wavelength restriction on the separation of the ground planeconductors, theattenuating card thickness is correspondingly limited. A further limitation of such prior art attenuating material is that the. attenuation provided by the granular resistive material varies linearly with the frequency. The present invention is directed to an improvement in such attenuators byproviding a solution for the problems arising from attempts to achieve a high degree of attenuation using an attenuating card of limited thickness and to achieve a relatively constant degree of attenuation over a range of frequencies.

It is, therefore, an object ofthe present provide an improved wave translating. device for high frequency transmission lines. 1

It is a further object of this-invention to provide an improved wave translating device exhibiting a high degree of attenuation of high frequency energy. 7 o

An additional object of the present invention is to provide an improved wave translating device exhibiting a relatively highdegree of attenuation over a wide range of high frequencies. 7 l V Yet another object of'this invention is to provide an improved high frequency attenuating material for use in high frequency attenuators.

In, accordance with the present invention, there is provided a unitary, composite, wave translating device for high frequency lines comprising'a pair of planar outer conductors providing ground planes. In insulated spaced relation between the outer conductors is a pair of eloninvention to 3,%,55 Patented July 24, 1%62 2 gated planar inner conductors adjacently disposed in different parallel planes. These inner conductors are connected together to operate electrically as a single conconductor.

ductor while substantially confining high frequency energy between each inner conductor and its adjacent outer Disposed intermediate the planes of the inner conductors is an attenuating means comprising a composite laminate of a metallic film layer and a layer of granular resistive material. Each of these layers provides a predetermined degree of attenuation of RF energy when placed in a field. The composite laminated material, however, exhibits a degree of attenuation in excess of the predetermined attenuations provided by either oi the layers.

Also in accordance with the present invention, there is provided a composite, laminated, high frequency attenuating material having a'layer of granular resistive material and a metallic film layer. Each of these layers provides a predetermined degree of attenuation of RF energy when placed in a field. The composite laminated material, however, exhibits a degree of attenuation in excess of the predetermined attenuations provided by either oi the layers.

For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description taken in connection with the accompanying drawings and its scope will be pointed out in the appended claims.

In the drawings:

FIG. 1 is a perspective view of acomposite, high frequency wave translating device of the present invention;

FIG. 2 is an elevational section of the wave translating device of FIG. ltaken along the line 24-2;

FIG. 3 is a cross-sectiona1 bottom view of the wave translating device of FIG. 1;

FIG. 4 is a perspective view of an attenuating card of the present'invention;

FIG. 5 is an elevational view in section of the attenuating card of FIG. 4 taken along the line 5--5;

FIG. 6 is an exploded view in perspective of the attenuating card of FIG. 4;

FIG. 7 is a schematic illustration of the electric field about a center conductor disposed between parallel ground planes;

FIG. 8 is a schematic illustration of the electric field about a spaced pair of center conductors disposed in register between parallel ground planes; and

FIG. 9 is a schematic illustration of FIG. 8 with the addition ofa planar attenuating member between and parallel to the center conductors.

Illustrated in FIGS. 1-3 is a wave translating device 11 for high frequency transmission lines which utilizes ar attenuating card such as the attenuating card 33 as more particularly illustrated in FIGS. 4-6. This device com prises a first fiat outer conductor 12 and a second fla' outer conductor 13 providing electrical ground planes An inner conductor assembly, for example, a pair of fiat elongated conductors 14, is formed into a loop or U shape and disposed in register in insulated spaced rela tion between the outer conductors 12 and 13. The at tenuating member here shown, e.g. as the attenuating card 33, is disposed between the inner conductor assem bly and is so rotatably mounted with respect to the formed by the inner conductors 14 as to introduce difier .ent areas of the card 33 between the inner conductor:

14' with different degrees of rotation of the card 33 Such rotation thereby provides a predetermined degret of attenuation of the high frequency electrical energ '33 between the conductors 14 by means of a rotatablt shaft 15 which is positioned by means of a knob 16 t indicate the degree of attenuation on a calibrated dial plate 17. The card may be rotated as indicated by the arrows 18 in FIG. 3 into the position shown by the phantom lines 19. A conductive means, here illustrated as a copper seal 20, surrounds the inner conductors 14 and the attenuator card 33 and connects the outer conductors 12 and 13 together to confine electric energy within the outer conductors.

An input transducer means, here shown ascoaxial connector 21, has its center conductor 22 connected to a pair of ends 23 of the inner conductors 14. An output transducer means, here shown as coaxial connector 24, has its inner conductor 25 connected to the other pair of ends 26 of the inner conductors 14.

Dielectric means, here shown as plastic insulating plates 27, 28 and 29, hold the conductors in insulated spaced relation as more particularly illustrated in FIG. 2. The plate 28 has a hole 30 formed therein to permit rotation of the card 33. The coaxial connectors 21 and 24 have their outer conductors connected to the outer conductors 12 and 13 by the conductive pins 31. The pins may be, for example, brass rivets or screws. The attenuator selecting shaft 15 is inserted through holes (not shown) in the various components and locked in place, for example, with a clamping ring 32. The center conductors 22 and 25 of the coaxial connectors 21 andv 24 are inserted through holes to contact the inner conductors 14 at the respective ends 23 and 26.

While applicant does not intend to be limited to any particular dimensions in the embodiment of the invention just described, there follows a set of dimensions which is found to be particularly suitable for devices of this character:

The over-all outside dimensions of the device 11 may be, for example, five inches square by inch thick. The ground planes 12 and 13 and the center conductors 14 may be made from one mil (0.001 inch) copper laminated to each side respectively of the dielectric layers 27 and 29. The over-all thickness of the ground plane-dielectric-inner conductor laminate may be, for example inch. Separating the ground plane-dielectricinner conductor laminates is the dielectric spacer 28 which has an aperture formed therein to receive the attenuating card 33. The dielectric material used in the card 33 may be, for example, polytetrafluoroethylene, such as Teflon, trademark of the E. I. du Pont de Nemours & Company. For convenience, the ground plane dielectric and inner conductors may be formed from commercially obtainable laminates of Teflon-Fiberglas bonded between sheets of one mil copper as, for example, the GB-112T Teflon- Fiberglas as manufactured by the Continental Diamond Fibre Division of the Budd Co., Inc., of Newark, Delaware. The inner conductor configuration may then be formed by conventional chemical etching techniques. The attenuating card 33 may be made, for example, of a polyethyleneterephthalate sheeting, such as Mylar, trademark of the E. I. du Pont de Nemours '& Co. of Wilmington, Delaware, metallized on one face and laminated between layers of carbonized fabric. This carbonized fabric may be, for example, USKON, trademark of the U. S. Rubber Company of Rockefeller Center, New York, N.Y. for their carbonized fabric, and may have a resistance of, for example, 377 ohms per square inch. This fabric consists of a linen cloth'with carbon granules embedded therein and bonded thereto with some suitable plastic binder in a manner well known in the art. The metallized Mylar may be /2 mil (0.0005 inch) Mylar sheeting "having a metallic aluminum film thereon as manufactured, for example, by National Metallizing Corp. of Newark, New Jersey, and may have a resistance of, for example, 6 ohms per square inch. The laminate may be'firmly bonded by means of an adhesive as, for example, a thermosetting epoxy adhesive such as Hysol Type 6020 I-Iardener C as made by Houghton Laboratories, inc. of Clean, New York. One layer of USKON and one layer of metallized Mylar may be used in place of the sandwich construction; however, the sandwich construction is preferred for reasons of symmetry. The adhesive bonding of the metallized Mylar between layers of the USKON fabric is not essential but it does tend to protect the metallic film which is very thin and fragile.

Referring now to FIGS. 4-6, there is here illustrated a laminated attenuating card 33 such as that described above. FIGS. 5 and 6 particularly show the carbonized fabric layers 34 and the metallic film layer 35.

The following table gives a comparison between the attenuation of a device having a laminated attenuating card and the general dimensions and character of the example described above with that of a similar device using a plain carbonized fabric attenuating card in place of the laminated card.

Attenuator Standard With Card of Attenuator Metalllzed With Plain Mylar Lami- Dial Setting USKON Card, nated Between 3,000 mc. USKON Attenuation Layers, 3,000

Inc. Attennation FIGS. 7-9 of the drawings are schematic illustrations of the mode of operation of the illustrated embodiment. FIG. 7 depicts the electric field lines present when a single center signal energy carrying conductor 14- is disposed between parallel ground planes 12 and 13, as is characteristic of the transverse electromagnetic 0r TEM mode of propagation. FIG. 8 shows the character of the field when two center conductors 14 in register and in separate parallel planes are connected electrically to operate as a single conductor. Note that fields which were previously terminated at the ends of the inner conductors, as shown in FIG. 7, here loop over into the space between the inner conductors. FIG. 9 shows the field configuration of the illustrated embodiment with the attenuating card disposed between the separated center conductors. Ordinarily it is a basic rule that the attenuating material be inserted in a plane parallel to the electric lines of force. Here the attenuating card is positioned tangential to the fringe field to distort and concentrate the field. This is, however, in effect equivalent to the insertion of an attenuating member at a point in the electric field parallel to the electric field. I

As pointed out above, the distance between the ground planes 12 and 13 must be less than one-fourth A) the wavelength at the highest operating frequency; therefore, the space available for the insertion of the attenuating card'33 is correspondingly limited. The laminated attenuating card structure described above provides a higher degree of attenuation over a wide range of high frequencies than any single material now known in the art. The theory of this is not entirely understood, as the characteristics of the individual materials comprising the laminate apparently do not combine to provide a composite material exhibiting a mathematically predictable degree of attenuation. In the case of the granular resistive material a decrease in the resistance of the material increases its attenuating ability. This is true only within a given range of resistance values, however, as there is a point after which an increase in conductivity will tend to decrease the attenuating ability of the granular material. Likewise, a metallic film exhibits increased attenuating ability as the resistance of the film is decreased. To exhibit any appreciable attenuating ability at all, however, a metallic film must have a thickness less than the curward in the art of wave translating devices by providing a degree of attenuation of high frequency energy not previously available.

While there has been described what is at present con- 7 sidered to be the preferred embodiment of this invention,

it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A unitary, composite, wave translating device for high frequency transmission lines, comprising: a pair of planar outer conductors providing ground planes; a pair of spaced, planar inner conductors adjacently disposed in diflerent planes in register, and in parallel with and in insulated spaced relation between said outer conductors, said inner conductors being connected together to operate electrically as a single conductor while substantially confining high frequency energy between each inner conductor and its adjacent outer conductor; and attenuating means disposed in a plane intermediate the planes of said inner conductors, said attenuating means comprising a composite laminate of a metallic film layer and a layer of granular resistive material, said metallic film layer'being of a thickness less than the penetrating depth of the energy transmitted along said transmission line.

2. A unitary, composite, wave translating device for high frequency transmission lines, comprising: a pair or": planar outer conductors providing ground planes; a pair of spaced, elongated, planar inner conductors adjacently disposed in different planes in register, and in parallel with and in insulated spaced relation between said outer conductors, saidinner conductors being spaced less than A wavelength apart and being connected together to operate electrically as a single conductor while substantially con fining high frequency energy between each inner conductor and its adjacent outer conductor; and attenuating means disposed in a plane intermediate the planes of said inner conductors, said attenuating means comprising a composite laminate of a metallic film layer having a thickness less than a current penetrating depth of the energy transmitted and a layer of granular resistive material.

3. A unitary, composite, wave translating device for high-frequency transmission lines, comprising: a pair 0; planar outer conductors providing ground planes; a pail of spaced, planar inner conductors adjacently disposed it different planes in register, and in parallel with and i1: insulated spaced relation between said outer conductors said inner conductors being connected together to operate electrically as a single conductor while substantially confining high frequency energy between each inner conductor and its adjacent outer conductor; and attenuating means disposed in a plane intermediate the planes of said inner conductors, said attenuating means comprising a composite laminate of a first layer of granular carbon material providing a predetermined degree of attenuation of RF energy when placed in a field, a second layer of granular carbon material providing a predetermined degree of attenuation of RF energy when placed in a field, and a layer of metallic film having a thickness less than the current penetrating depth of the energy transmitted, said composite laminate being less than wavelength thick.

4. A unitary, composite, wave translating device for high frequency transmission lines, comprising: a first flat outer conductor providing an electrical ground plane; a second fiat outer conductor providing an electrical ground plane; a pair of flat, elongated, inner conductors formed into a pair of identical parallel loops connected together to operate electrically as a single conductor and disposed in register in difierent planes parallel to and in insulated spaced relation between said outer conductors; and attenuator material disposed between said inner conductors and so rotatably mounted with respect to said loops as to introduce different areas of said attenuation material between said inner conductors with difierent degrees of rotation of said material to provide a pre-determined degree of attenuation to the passing of electric energy through said attenuator, said attenuating material comprising a composite laminate of a first layer of resistive material, a second layer of resistive material, and a layer of metallic film having a thickness less than the penetrating depth of the energy transmitted, said layer of film being disposed between said first and second layers of resistive material.

References Cited in the file of this patent UNITED STATES PATENTS 2,405,449 Robinson Aug. 6, 1946 2,515,228 Hupcey July 18, 2,610,250 Wheeler Sept. 9, 1952 2,664,453 Lang Dec. 29, 1952 2,702,580 Bateman Feb. 22, 1955 2,796,588 Walker June 18, 1957 2,832,713 Ragan Apr. 29, 195% 2,883,315 Palmquist Apr. 21, 1955 2,909,736 Sommers et al. Oct. 20, 1955 

